1. Field of the Invention
This invention relates to an automatic amplitude equalizer which compensates for an amplitude characteristic of an input signal.
2. Description of the Related Art
In recent years, in digital multiplex radio apparatus, in order to improve the signal quality of a channel against deterioration arising from a transmission line distortion such as a fading distortion which occurs in the space, transversal equalizers which can equalize the transmission line distortion in the time-domain are practically used widely.
It is known, however, that the equalization characteristic (also called signature characteristic) of a transversal equalizer, for example, of the orthogonal two-dimensional type is called "M curve" and is inferior in primary first-order inclination characteristic in an IF (intermediate frequency) band.
Therefore, it is demanded to use, in addition to a transversal equalizer which equalizes a transmission line distortion (first-order inclination distortion) of a first-order inclination characteristic of an IF signal (input signal) in the time-domain, an equalizer which equalizes the transmission line distortion in the frequency-domain or the time-domain to compensate for the transmission line distortion more effectively.
Also it is demanded for an equalizer to effectively detect a secondary or second-order inclination distortion of an IF signal and compensate for not only a first-order inclination distortion of the IF signal but also a second-order inclination distortion of the IF signal effectively to assure a higher performance of an equalizer.
FIG. 65 is a block diagram showing a construction of an ordinary automatic amplitude equalizer. Referring to FIG. 65, the automatic amplitude equalizer shown includes a first-order inclination compensation section 100, an automatic gain control section (AGC) 200, a three-wave detector 300 and a comparison circuit 400.
The first-order inclination compensation section 100 has a first-order inclination amplitude characteristic, which is controlled in accordance with a control signal from the comparison circuit 400 which will be hereinafter described so that the first-order inclination compensation section 100 compensates for a first-order inclination distortion of a received signal transmitted thereto in the space in accordance with the first-order inclination characteristic thereof. The automatic gain control section 200 controls the gain of the output of the first-order inclination compensation section 100 fixed so that a circuit at a following stage to the first-order inclination compensation section 100 such as a demodulator may operate regularly.
The three-wave detector 300 performs three-wave detection for the output of the automatic gain control section 200 to detect three different frequency components f.sub.0, f.sub.1.sup.- and f.sub.2.sup.+ of the output. The comparison circuit 400 compares the three frequency components f.sub.0, f.sub.1.sup.- and f.sub.2.sup.+ obtained from the three-wave detector 300 with each other to detect a first-order inclination distortion of the input signal (received signal) of the equalizer and outputs a control signal for the first-order inclination compensation section 100 for compensating for the first-order inclination distortion.
In the automatic amplitude equalizer having the construction described above, an input signal to the equalizer is controlled fixed in gain by the automatic gain control section 200 and three-wave detected by the three-wave detector 300. The three frequency components f.sub.0, f.sub.1.sup.- and f.sub.2.sup.+ thus obtained from the three-wave detector 300 are compared with each other by the comparison circuit 400 to detect a first-order inclination distortion of the input signal.
Then, from the comparison circuit 400, a control signal for controlling the first-order inclination amplitude characteristic of the first-order inclination compensation section 100 is outputted in order to compensate for the first-order inclination distortion. In accordance with the control signal, the first-order inclination compensation section 100 compensates for the first-order inclination characteristic of the input signal.
FIG. 66 is a block diagram showing a construction of another ordinary automatic amplitude equalizer. Referring to FIG. 66, the automatic amplitude equalizer shown includes a reception section 10, a first-order inclination compensation section 20', a variable gain amplification section 30, a demodulation section 40 including a transversal equalizer (TRE) 41, an identification section 50, an amplitude detection section 60, and a control section 90'. It is to be noted that reference numeral 101 denotes an antenna.
The reception section 10 down converts a RF (radio frequency) signal received by the antenna 101 into an IF (intermediate frequency) signal. The first-order inclination compensation section 20' has a first-order inclination amplitude characteristic and compensates for a first-order inclination distortion of an IF signal in accordance with the first-order inclination amplitude characteristic. The first-order inclination compensation section 20' include a distributor (hybrid type) 211, a positive inclination first-order amplitude equalization section 212 which makes use of a positive inclination first-order characteristic of a notch filter or a like element, a negative inclination first-order amplitude equalization section 213 which similarly makes use of a negative inclination first-order characteristic of a notch filter, a pair of variable attenuators 214 and 215 for which a PIN diode or a like element is employed, and a mixer (hybrid type) 216.
The variable gain amplification section 30 controls the amplification degree of the output of the first-order inclination compensation section 20' in accordance with an AGC (Automatic Gain Control) signal from the amplitude detection section 60, which will be hereinafter described, so that the gain of the output thereof to the demodulation section 40 may be fixed. The demodulation section 40 demodulates the output of the variable gain amplification section 30 using a suitable demodulation method such as orthogonal detection to obtain a demodulated base band signal (BBS). The demodulation section 40 is constructed including the transversal equalizer 41, for example, of the 7-tap type.
The identification section 50 identifies a demodulated base band signal obtained from the demodulation section 40 with a required identification level. The amplitude detection section 60 compares the BBS signal from the demodulation section 40 with a predetermined reference value (symbol level) in synchronism with a symbol timing clock signal (SCK) to produce an AGC signal for automatically controlling the gain of the variable gain amplification section 30.
The control section 90' detects a first-order inclination distortion of an IF signal and produces based on the thus detected first-order inclination distortion and outputs to the first-order inclination compensation section 20' a control signal for controlling the mixing ratio between the outputs of the positive inclination first-order amplitude equalization section 212 and the negative inclination first-order amplitude equalization section 213 in the first-order inclination compensation section 20'. The control section 90' includes the spectrum distortion detection section 70' for detecting a first-order inclination distortion in a spectrum of an IF signal, and the mixing ratio control section 80' for producing a control signal in response to a result of detection by the spectrum distortion detection section 70'.
In the automatic amplitude equalizer having the construction described above, a RF (radio frequency) signal received by the antenna 101 is amplified and down converted into an IF (intermediate frequency) signal having a center frequency at f.sub.0, which will be hereinafter described with reference to FIG. 67(a), by the reception section 10. Then, the IF signal is compensated for, in first-order inclination distortion thereof, by the first-order inclination compensation section 20', and an IF.sub.EQ signal after compensation is outputted from the first-order inclination compensation section 20'.
The IF.sub.EQ signal is amplified by the variable gain amplification section 30 and then inputted to the demodulation section 40, in which the input signal (IF.sub.EQ signal) is demodulated using a predetermined demodulation method such as orthogonal detection to obtain a demodulated base band signal (BBS).
Then, the demodulated base band signal BBS is identified with a required identification level by and outputted as a received data from the identification section 50. Meanwhile, the amplitude detection section 60 compares the base band signal (BBS) outputted from the demodulation section 40 with a predetermined reference value (symbol level) in synchronism with a symbol timing clock signal SCK to perform automatic gain control (AGC) so that the input signal level to the demodulation section 40 may always be fixed.
The demodulation section 40 described above includes the transversal equalizer 41 so that a fading distortion of an IF signal may basically be corrected in the time-domain. For the transversal equalizer 41, a transversal equalizer of the 7-tap type having, for example, such a signature characteristic (M curve) as represented by a characteristic T.sub.7 in FIG. 67(a) is employed normally. It is to be noted that, in FIG. 67(a), the axis of abscissa represents the fading frequency (MHz), and the axis of ordinate represents the depth of fading (dB).
However, if it is tried to realize a uniform error rate of approximately 10.sup.-3 over the entire IF band using a transversal equalizer of the 7-tap type, when a distortion (deterioration in amplitude) by fading occurs in the proximity of the frequency f.sub.0, the distortion can be compensated for (equalized) to the depth of up to approximately 17 dB, but when a distortion by fading occurs at any of the opposite shoulder portions (first-order inclination), the distortion can be compensated for only to the depth of up to approximately 15 dB. The difference between the depths is approximately 2 dB, and the inclination of the characteristic T.sub.7 is comparatively moderate.
Thus, it has been devised and put into practical use to compensate for an amount of deterioration in amplitude at any of the opposite shoulder portions of such a characteristic T.sub.7 as illustrated in FIG. 67(a) separately by means of a first-order inclination compensation section which has such a first-order inclination amplitude equalization characteristic as the first-order inclination compensation section 20' described above.
In particular, in this instance, an IF signal is branched by the distributor 211 into two waves, which are individually inputted to the positive inclination first-order amplitude equalization section 212 and the negative inclination first-order amplitude equalization section 213 which have first-order inclination amplitude equalization characteristics of a positive inclination and a negative inclination, respectively. The positive inclination first-order amplitude equalization section 212 performs first-order amplitude equalization of the positive inclination for the input signal thereto in the frequency-domain while the negative inclination first-order amplitude equalization section 213 performs first-order amplitude equalization of the negative inclination for the input signal thereto in the frequency-domain.
The outputs of the two equalization sections 212 and 213 are attenuated by the variable attenuators 214 and 215, respectively, so that they have such a mixing ratio with which the inclination amplitude distortion of the received signal may be cancelled, and are then mixed (composed) by the mixer 216. An example of operation in this instance will be described in detail below.
First, the spectrum distortion detection section 70' detects an amount of a first-order inclination distortion in a spectrum by two-point detection of frequencies (f.sub.0 -.increment.f, f.sub.0 +.increment.f) at the opposite shoulder portions of the IF band. Further, if the the spectrum distortion detection section 70' detects a distortion of a positive inclination from the IF.sub.EQ signal, then it outputs a negative detection signal SPD, but if it detects a distortion of a negative inclination, then it outputs a positive detection signal SPD. Then, the mixing ratio control section 80' integrates, in the inside thereof, the positive or negative detection signal SPD to produce a distortion detection signal.
In this instance, the distortion detection signal is driven to the "0" side when the spectrum of the IF.sub.EQ signal is flat or has a distortion in the proximity of the frequency f.sub.0, but is driven to the "-" side when the spectrum of the IF.sub.EQ signal has a distortion of a positive inclination, whereas it is driven to the "+" side when the spectrum of the IF.sub.EQ signal has a distortion of a negative inclination. In response to the distortion detection signal, the control signals to be applied to the variable attenuators 214 and 215 are varied symmetrically in such manners as seen from curves "a" and "b" in FIG. 67(b).
It is to be noted that, in this instance, the variable attenuators 214 and 215 have such a characteristic that, when the control voltage "a" or "b" is low, the attenuation amount approaches ".infin.", but on the contrary when the control voltage "a" or "b" is high, the attenuation amount approaches "0".
FIG. 68(a) is a diagram illustrating an example of equalization operation of the first-order inclination compensation section 20'. Referring to FIG. 68(a), the characteristic p indicates a positive inclination first-order amplitude equalization characteristic of the positive inclination first-order amplitude equalization section 212, and the characteristic n indicates a native inclination first-order amplitude equalization characteristic of the negative inclination first-order amplitude equalization section 213. A composite characteristic of the characteristics p and n is indicated by the characteristic m. It is to be noted that, in this instance, a mixing ratio between the characteristics p and n is taken into consideration, and in the example shown, the mixing ratio is p:n=1:2.
The composite characteristic m is, as seen in FIG. 68(a), opposite to the positive inclination distortion of the input IF signal, and consequently, if the IF signal is equalized in amplitude with the composite characteristic m, then a flat IF.sub.EQ signal is obtained at the output of the first-order inclination compensation section 20'. The first-order inclination compensation section 20' is constructed such that this relationship stands for any first-order inclination distortion of the IF signal. Accordingly, in the system in which the first-order inclination compensation section 20' is employed in addition to the transversal equalizer 41 of the 7-tap type described above, also the fading distortion at any of the opposite shoulder portions of the IF band is compensated for to the depth of 17 dB similarly to that at a central portion of the IF band, and consequently, the M curve exhibits a flat configuration (not shown).
A further automatic amplitude equalizer which compensates a first-order inclination distortion of an input signal is disclosed in Japanese Patent Laid-Open Application No. show a 58-198928 wherein a first-order inclination distortion of a received signal is detected from two different analog signals (I, Q) obtained by demodulation of the input signal and orthogonal to each other to compensate for the first-order inclination distortion of the received signal to equalize the amplitude of the received signal.
While several automatic amplitude equalizers are described above, the automatic amplitude equalizer shown in FIG. 65 has a subject to be solved in that a large circuit scale and a high cost are required because, upon detection of a first-order inclination distortion, the frequency components f.sub.0, f.sub.1.sup.- and f.sub.2.sup.+ which are to be used for comparison by the comparison circuit 400 are detected using the three-wave detector 300 which has a large circuit scale and is expensive.
Meanwhile, an automatic amplitude equalizer of the type shown in FIG. 66 nowadays employs, for example, in place of the transversal equalizer 41, a transversal equalizer of the 9-tap type having such an M curve as indicated by a characteristic T.sub.9 in FIG. 67(a), and consequently is improved very much in equalization characteristic.
If it is tried to realize a uniform error rate of approximately 10.sup.-3 over the entire IF band using a transversal equalizer of the 9-tap type just mentioned, then when fading occurs in the proximity of the frequency f.sub.0, a first-order inclination distortion of an input signal can be compensated for to the depth of up to approximately 20 dB, but when fading occurs at any of the opposite shoulder portions (first-order inclination) of the IF band, the first-order inclination distortion of the input signal can be compensated for to the depth of up to approximately 16 dB. In short, the difference between the depths is approximately 4 dB, and the inclination of the characteristic T.sub.9 is steeper than that of the characteristic T.sub.7.
However, if it is tried to compensate for such a steep characteristic T.sub.9 by means of the first-order inclination compensation section 20', then a steep equalization characteristic is required for each of the positive inclination first-order amplitude equalization section 212 and the negative inclination first-order amplitude equalization section 213 accordingly. However, an amplitude characteristic which satisfies such a steep equalization characteristic is rather near to a second- or third-order characteristic, and if a first-order characteristic is used as approximate characteristic to the amplitude characteristic, then the following problem is invited.
In particular, referring to FIG. 68(b), the characteristic p represents a positive inclination second-order equalization characteristic of the positive inclination first-order amplitude equalization section 212, and the characteristic n represents a negative inclination second-order equalization characteristic of the negative inclination first-order amplitude equalization section 213. It is to be noted that, in FIG. 68(b), each of the second-order characteristics is represented approximately as a polygonal line. Then, the composite characteristic of the characteristics p and n is indicated by m. If an IF signal having a positive inclination distortion similar to that illustrated in FIG. 68(a) is inputted to the first-order inclination compensation section 20', then the amplitude of the IF.sub.EQ signal at the output of the first-order inclination compensation section 20' exhibits deterioration in the proximity of the frequency f.sub.0 as seen in FIG. 68(b). This phenomenon always occurs if the inclinations of the characteristics p and n are steep.
Therefore, in such a system as employs the first-order inclination compensation section 20' in addition to a transversal equalizer of the 9-tap type, the overall equalization characteristic EQT.sub.9 of the M curve does not exhibit such a flat configuration as seen in FIG. 67(a), but rather exhibits deterioration of approximately 3 dB in the proximity of the frequency f.sub.0.
On the other hand, the apparatus disclosed in Japanese Patent Laid-Open Application No. show a 58-198928 still has a subject to be solved in that the circuit scale or the cost of an apparatus cannot be reduced because also a detection system for detecting a first-order inclination distortion of an input signal from analog signals (I, Q) obtained by demodulation of the input signal is formed from an analog circuit.
Further, although it is a common practice to detect a first-order inclination distortion of an input signal and compensate for the first-order inclination distortion of the input signal based on the thus detected first-order inclination distortion to equalize the input signal, it is not a common practice to detect a second-order inclination distortion (second-order inclination amplitude characteristic) of an input signal to compensate for the second-order inclination distortion.